The increasing demand for wood products and wood derived products constitutes a problem of global proportion. It is estimated that the maximum sustainable rate of harvesting from the world's forests has already been reached. Thus, there is an imminent need for more woody plants, as well as a need for developing methods for increasing the agronomic properties of forestry plants, such as enhanced plant height, enhanced biomass production, and longer xylem fiber length. For example, fiber uniformity and strength are common requirements for most industrial uses. In pulp manufacture, strength characteristics are determined in part by fiber length. Long fibers are ideal for strong paper production, pulp yield increase and decrease in alkali consumption, due to their strength and bonding properties.
As an illustrative example of the importance of woody plants, one can mention Eucalyptus trees, which represent the largest sources of fibers used globally in the paper industry. Bamber, 1985, Appita 38: 210-216). There are an estimated ten to fifteen million hectares of land planted with Eucalyptus. Verhaegen and Plomion, 1996, Genome 39: 1051-1061. The major advantage of the Eucalyptus tree is its very high growth rate and ability to grow in a wide range of conditions, both tropical and temperate. The Eucalyptus fibers have one disadvantage, however, compared to fibers from other sources, such as pine, which is their significantly shorter length. Thus, papers that are made from Eucalyptus pulp are often weak and usually require reinforcement with longer fibers from other sources increasing the production costs.
Fiber length is controlled by endogenous regulation of cell elongation, a process which results from the interaction between internal turgor pressure and the mechanical strength of the cell wall, but its mechanism and genes involved have not been yet totally discerned.
Xylem fiber cells develop from already much-elongated fusiform initials located within the vascular cambium. They increase in diameter by extension of their radial walls, and, in addition, developing fiber cells elongate by intrusive tip growth, which results in up to a severalfold increase in cell length. Gray-Mitsumune et al., 2004, Plant Physiol. 135: 1552-1564.
In tip-growing cells, expansion occurs over a small area of the cell surface, which results in tubular, elongated cells. For example, poplar fibers elongate intrusively in the radial-expansion zone in the xylem, reaching 150% of their initial cell length at the average when fully differentiated. Hussey et al., 2006, Annu. Rev. Plant Biol. 57: 109-125; Mellerowicz et al., 2001, Plant Mol. Biol. 47: 239-274.
The rapid expansion of fiber cells may be achieved by concerted action of pushing against the cell wall exerted by turgor and loosening of the cell wall. In cotton fibers, the phase of cell elongation follows a significant rise of turgor, resulted from the observed accumulation of malate, sugars, and K+, the major osmoticum, hence the influx of water and the generation of high turgor in the fiber cells. Ruan et al., 2004, Plant Physiol. 136: 4104-4113.
Vacuolar invertases can play an important role in turgor maintenance and cell wall expansion. Recent work in Arabidopsis thaliana has shown that a wall-associated kinase (WAK) can regulate a vacuolar invertase thus establishing a cross-compartmental link between WAK and vacuolar invertase(s). Kohorn et al., 2006, Plant J. 46: 307-316.
In Arabidopsis WAKs are encoded by five tightly linked and highly similar genes, and are expressed in leaves, meristems, and cells undergoing expansion. Wagner and Kohorn, 2001, Plant Cell 13: 303-318.
Mutant seedlings of Arabidopsis thaliana presenting a T-DNA insertion in the WAK2 gene were significantly shorter than wild-type plants, with the roots more affected than the hypocotyls. Kohorn et al., 2006, Plant J. 46: 307-316.
These mutant plants showed a reduced vacuolar invertase activity by 62%, and the authors proposed that WAK2 regulates the transcription of vacuolar invertase as one constituent of a mechanism modulating solute concentrations and turgor regulation, thus providing a possible mechanism for WAK to regulate cell expansion.
The expression of an inducible antisense WAK2 in Arabidopsis led to a 50% reduction in WAK protein levels, with a subsequent loss of cell elongation, and hence dwarf plants. Similar results have been reported when an antisense WAK4 gene was used to reduce total WAK protein levels. Wagner and Kohorn, 2001, Plant Cell 13: 303-318; Lally et al., 2001, Plant Cell 13: 1317-1331.
It is also known that the wall-associated kinases contain extracellular domains that can be linked to pectin molecules of the cell wall, span the plasma membrane and have a cytoplasmic serine/threonine kinase domain. He et al., 1999, Plant Mol. Biol. 39: 1189-1196.
When fibers undergo significant elongation at both ends (intrusive tip growth), the properties of the middle lamella limit this type of cell growth. Middle lamellae of developing wood cells are rich in pectins, and intrusive tip growth requires the dissolution of the middle lamella. See Berthold et al., WO 2006/068603.
By their pectin attachment, it is possible that WAKs may sense a change in the cell wall environment, thus providing a molecular mechanism linking cell wall sensing to regulation of solute metabolism, which in turn is known to be involved in turgor maintenance and cell expansion in growing cells. Such information could be invaluable to adjustment of cell expansion or turgor. Huang et al., 2007, Functional Plant Biology, 34: 499-507.
Fiber characteristics are controlled by a complex set of genetic factors and are not easily amenable to classical breeding methods. Through traditional forest tree breeding it is possible to achieve some modification of fiber characteristics. For example, interspecific triploid hybrids of poplar have been developed which have longer fibers than the parental species. Aziz et al., 1996, Wood and pulp properties of aspen and its hybrids. TAPPI Proc. Pulping Conference. p. 437-443. Yet, considering the disadvantage of traditional forest tree breeding, such as the slow progress due to their long generation periods and the difficulty of producing a plant with a desirable trait, the developments in gene technology can reduce significantly the time required to produce a new variety of plant and allow closer targeting of traits considered desirable by the forest and pulp industries in specific trees species.